Abstract

Books and other paged media often use special techniques to display information. Content may be moved to or generated for special areas of the page, such as running heads or footnotes. Generated content within pages, such as tab leaders or cross-references, helps readers navigate within and between pages.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This draft updates the earlier draft of GCPM and, where content has been moved to other modules, an appendix indicates the current location of each.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

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Introduction

Paged media have many special requirements for the display of document content, which have evolved over the long history of printed books. Running headers and footers function as aids to navigation. Notes may appear at the bottom of the page, as footnotes. The properties of pages themselves may change based on their content or position in the document. Leaders visually connect related content. Cross-references may require generated text. Some paged media formats such as PDF use bookmarks for navigation.

This module defines new properties and values, so that authors may bring these techniques to paged media.

1
Running headers and footers

[CSS3PAGE] describes the sixteen page margin boxes which can be used for running headers and footers, but does not describe a mechanism for inserting content in those boxes. This module provides two methods for for doing so. Named strings copy text for reuse in margin boxes. Running elements move elements (complete with style and structure) from the document to margin boxes.

1.1
Named strings

The string-set property copies the text content of an element into a named string, which functions as a variable. The text content of this named string can be retrieved using the string() function. Since these variables may change on a given page, an optional second value for the string() function allows authors to choose which value on a page is used.

The string-set property contains one or more pairs, each consisting of an custom identifier (the name of the named string) followed by a content-list describing how to construct the value of the named string.

content-list expands to one or more of the following values, in any order.

The string value of the element, determined as if white-space: normal had been set. This is the default value

before

The string value of the ::before pseudo-element, determined as if white-space: normal had been set.

after

The string value of the ::after pseudo-element, determined as if white-space: normal had been set.

first-letter

The first letter of the element, as defined for the ::first-letter pseudo-element

The content values of named strings are assigned at the point when the content box of the element is first created (or would have been created if the element’s display value is none). The entry value for a page is the assignment in effect at the end of the previous page. The exit value for a page is the assignment in effect at the end of the current page.

Whenever the value of the element changes, the value of the named string(s) is updated. User agents must be able to recall many values of the named string, as the string() function can return past, current, or future values of the assignment.

The string() function is used to copy the value of a named string to the document, via the content property. This function requires one argument, the name of the named string. Since the value of a named string may change several times on a page (as new instances of the element defining the string appear) an optional second argument indicates which value of the named string should be used.

The following figures show the first, start, and last assignments of the “heading” string on various pages.

The start value is empty, as the string had not yet been set at the start of the page.Since the page starts with an h2, the start value is the value of that head.Since there’s not an h2 at the top of this page, the start value is the exit value of the previous page.

1.2
Running elements

Many headers and footers cannot be represented only by unformatted text. A book title serving as a running head may contain an italic word, for example. A mechanism is needed to move or copy elements into margin boxes. To do this, we add the running() value to the position property, and the element() value to the content property.

position: running(custom-ident) removes the element (and associated ::before and ::after pseudo-elements) from the normal flow, and makes it available to place in a page margin box using element(). The element inherits from its original position in the document, but does not display there.

The element value() can only be used in page margin boxes. And it cannot be combined with other possible values for the content property.

This idea would be much more useful if we could also copy (rather than just move) elements. That would avoid the duplication of HTML in the example above.

Bert Bos has proposed an alternative syntax, which allows both moving and copying elements into running heads. In the example below, h2 elements appear in their normal place in the document, but are also copied into running heads.

The element() value of the content property places an element (which has been removed from the normal flow via running()) in a page margin box. Whenever the value of the element changes, the value of element() is updated.

Just as with string(), element() takes an optional keyword to describe which value should be used in the case of multiple assignments on a page. User agents must be able to recall many values, as element() can return past, current, or future values of the assignment.

2.1 Terminology

The element containing the content of the footnote, which will be removed from the flow and displayed as a footnote.

footnote marker (also known as footnote number)

A number or symbol adjacent to the footnote body, identifying the particular footnote. The footnote marker should use the same number or symbol as the corresponding footnote call, although the marker may contain additional punctuation.

footnote body

The footnote marker is placed before the footnote element, and together they represent the footnote body, which will be placed in the footnote area.

footnote call (also known as footnote reference)

A number or symbol, found in the main text, which points to the footnote body.

footnote area

The page area used to display footnotes.

footnote rule (also known as footnote separator)

A horizontal rule is often used to separate the footnote area from the rest of the page. The separator (and the entire footnote area) cannot be rendered on a page with no footnotes.

2.2
Creating footnotes

An element becomes a footnote by applying float: footnote to that element. This triggers the following actions:

The footnote element is removed from the flow, and a ::footnote-call pseudo-element is inserted in its place, which serves as a reference to the footnote.

A ::footnote-marker pseudo-element, identifying the footnote, is placed at the beginning of the footnote element. Together this is the footnote body.

The footnote body is placed in the footnote area at the bottom of the page. Footnote elements from a given page are placed in the footnote area of that page in document order.

HTML:

<p>Though the body was erect, the head was thrown back so that the closed eyes were pointed towards the needle of the tell-tale that swung from a beam in the ceiling.<span class="footnote">The cabin-compass is called the tell-tale, because without going to the compass at the helm, the Captain, while below, can inform himself of the course of the ship.</span></p>

Why is float:bottom used with the footnote area? Floating footnotes to the footnote area, and then floating the footnote area itself, seems overly complex, given that implementations don’t allow the footnote area to float anywhere else. Note that some implementations do allow the footnote area to be absolutely positioned.

2.3
Types of footnotes

The following new value of the float property creates a footnote element:

The footnote-display property determines whether a footnote is displayed as a block element or inline element.

Name:

footnote-display

Value:

block | inline | compact

Initial:

block

Applies to:

elements

Inherited:

no

Media:

paged

Computed value:

specified value

Percentages:

N/A

block

The footnote element is placed in the footnote area as a block element

inline

The footnote element is placed in the footnote area as an inline element

compact

The user agent determines whether a given footnote element is placed as a block element or an inline element. If two or more footnotes could fit on the same line in the footnote area, they should be placed inline.

2.4
The footnote area

A page area that can be used to display footnotes is described in the page context using an @footnote rule. This rule defines a box that, if used, will contain all the footnote elements that appear on that page.

How would one describe this in the grammar of CSS3-Page?

2.4.1 Positioning of the footnote area

The bottom margin edge of the footnote area is positioned so that it touches the bottom of the page area. The footnote area can only contain footnotes.

How do footnotes work in multi-column text? Prince uses float: prince-column-footnote to create a footnote at the bottom of a column rather than the bottom of a page.

Implementations that support footnotes generally support page floats like float: bottom. Page floats should end up above the footnote area. How might this be specified?

2.4.2 Size of the footnote area

The max-height property on the footnote area limits the size of this area, unless the page contains only footnotes (as may happen on the last page of a document).

Since it is undesirable for a page to consist only of footnotes, user agents may set a default max-height value on the footnote area.

2.5 The Footnote Counter

The footnote counter is a predefined counter associated with the footnote element. Its value is the number or symbol used to identify the footnote. This value is used in both the footnote call and the footnote marker. It should be incremented for each footnote.

2.5.1 Values of the footnote counter

The footnote counter, like other counters, may use any counter style. Footnotes often use a sequence of symbols.

2.5.2 Resetting the footnote counter

The footnote counter may be reset on each page.

@page {
counter-reset: footnote;
@footnote { … }
}

Note that the value of the footnote counter should depend on the position of the footnote element in the document tree, not where it is eventually placed. A footnote element may sometimes be placed on the page after the footnote call, but the same counter value must be used for both.

2.6
The footnote-call pseudo-element

A ::footnote-call pseudo-element is inserted in place of the footnote element when the latter is removed from the flow. By default, the content of this pseudo-element is the value of the footnote counter, styled as a superscripted number.

2.7
The footnote-marker pseudo-element

The ::footnote-marker pseudo-element represents the footnote element’s marker, the number or symbol that identifies each footnote. This pseudo-element behaves like a ::marker pseudo-element, as defined in [CSS3LIST]. It is placed at the beginning of the superior parent’s content, and is inline by default. The ::footnote-marker can be styled just as other ::marker elements can be. The default style should include list-style-position: inside.

2.8
Rendering footnotes and footnote policy

Rendering footnotes can be complex. If a footnote falls near the bottom of the page, there may not be enough room on the page for the footnote body. The footnote-policy property allows authors some influence over the rendering of difficult pages.

Name:

footnote-policy

Value:

auto | line | block

Initial:

auto

Applies to:

elements

Inherited:

no

Media:

paged

Computed value:

specified value

Percentages:

N/A

auto

The user agent chooses how to render footnotes, and may place the footnote body on a later page than the footnote reference. A footnote body must never be placed on a page before the footnote reference.

line

If a given footnote body cannot be placed on the current page due to lack of space, the user agent introduces a forced page break at the start of the line containing the footnote reference, so that both the reference and the footnote body fall on the next page. Note that the user agent must honor widow and orphan settings when doing this, and so may need to insert the page break on an earlier line.

block

As with line, except a forced page break is introduced before the paragraph that contains the footnote.

We need an algorithm for laying out footnotes

2.9
Future directions

The next level will include sidenotes, column footnotes, and multiple footnote areas.

3
Selecting Pages

A paginated document consists of a sequence of pages. [[CSS3PAGE]] defines page selectors, which allow the selection of the first page of the document, left and right pages, and blank pages. Here we extend the idea of page selectors to enable the selection of arbitrary document pages.

3.1
Page Selectors

The nth() page pseudo-class allows the selection of arbitrary document pages. This pseudo-class takes an argument of the form An + B as defined in [CSS3SYN]. When applied to the default @page rule, nth() selects the document page whose index matches the argument.

nth() is not related to the page counter, which may reset and use various numbering schemes.

When the nth() selector is applied to a named page, and that named page is part of a page-group (see below), it selects the nth page in the page group.

@page :nth(1)

This selects the first page of the document.

@page :nth(2n)

This selects all even document pages.

3.2
Page groups

Many paginated documents have a repeating structure, consisting of many chapters, sections, or articles. The first page of each subdivision often requires special treatment, but [CSS3PAGE] doesn’t define a way to select the first page of each chapter (as distinct from the first page of the entire document).

When the page property is applied to an element that also has a forced break property applied, a page group is created. The page group is the collection of all pages created by an instance of that element. When a new instance of the element is rendered, a new page group is started.

A page may be part of several page groups, as a given page group may have ancestors or descendants that are part of another page group.

CSS:

div { page: A }
child { page: B }

A page may be part of several page groups.
Note that page 5 can be selected in three ways:

@page :nth(5 of A) /* will select 5th page of every <div> */
@page :nth(1 of B) /* will select first page of every <child> */
@page :nth(5) /* will select 5th page of document */

In this case, each chapter will form a separate page group. @page:nth(3 of body) will select the third page of each chapter, even if that page happens to use the “broadside” named page. @page:nth(1) will select only the first page of the document, as will @page:first.

4
Leaders

A leader, sometimes known as a tab leader or a dot leader, is a repeating pattern used to visually connect content across horizontal spaces. They are most commonly used in tables of contents, between titles and page numbers. The leader() function, as a value for the content property, is used to create leaders in CSS. This function takes a string (the leader string), which describes the repeating pattern for the leader.

Do leaders depend on the assumption that the content after the leader is right-aligned (end-aligned)?

4.1
Rendering leaders

Consider a line which contains the content before the leader (the “before content”), the leader, and the content after the leader (the “after content”). Leaders obey the following rules:

The leader string must appear in full at least once.

The leader should be as long as possible

Visible characters in leaders should vertically align with each other when possible.

Line break characters in the leader string must be ignored.

White space in the leader string follows normal CSS rules.

A leader only appears between the start content and the end content.

A leader only appears on a single line, even if the before content and after content are on different lines.

A leader can’t be the only thing on a line.

4.1.1
Procedure for rendering leaders

Lay out the before content, until reaching the line where the before content ends.

BBBBBBBBBB
BBB

The leader string consists of one or more glyphs, and is thus an inline box. A leader is a row of these boxes, drawn from the end edge to the start edge, where only those boxes not overlaid by the before or after content. On this line, draw the leader string, starting from the end edge, repeating as many times as possible until reaching the start edge.

BBBBBBBBBB
..........

Draw the before and after content on top of the leader. If any part of the before or after content overlaps a glyph in a leader string box, that glyph is not displayed.

BBBBBBBBBB
BBB....AAA

If one full copy of the leader string is not visible:

BBBBBBB
BBBBBBA

Insert a line break before the after content, draw the leader on the next line, and draw the end content on top

BBBBBBB
BBBBBB
......A

Procedure for drawing leadersProcedure for drawing leaders when the content doesn’t fit on a single line

5
Cross-references

Many documents contain internal references:

See chapter 7

in section 4.1

on page 23

Three new values for the content property are used to automatically create these types of cross-references: target-counter(), target-counters(), and target-text(). Each of these displays information obtained from the target end of a link.

The target-counter() function retrieves the value of the innermost counter with a given name. The required arguments are the url of the target and the name of the counter. An optional counter-style argument can be used to format the result.

These functions only take a fragment URL which points to a location in the current document. If there’s no fragment, if the ID referenced isn’t there, or if the URL points to an outside document, the user agent must treat that as an error.

5.3
target-text

The target-text() function retrieves the text value of the element referred to by the URL. An optional second argument specifies what content is retrieved, using the same values as the string-set property above.

Result: …which will be discussed later, in the chapter entitled Loomings.

6
Bookmarks

Some document formats, most notably PDF, allow the use of bookmarks as an aid to navigation. Bookmarks provide a hierarchy of links to document sections, as well as text to label the links. A bookmark has three properties: bookmark-level, bookmark-label, and bookmark-state.

6.1
bookmark-level

Name:

bookmark-level

Value:

none | <integer>

Initial:

none

Applies to:

all elements

Inherited:

no

Media:

paged

Computed value:

specified value

Percentages:

N/A

<integer>

defines the level of the bookmark, with the highest level being 1 (negative and zero values are invalid).

Conformance

Document conventions

Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.

All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with class="example",
like this:

This is an example of an informative example.

Informative notes begin with the word "Note" and are set apart from the
normative text with class="note", like this:

Note, this is an informative note.

Conformance classes

Conformance to this specification
is defined for three conformance classes:

A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.

A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must
treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.

Issues Index

This idea would be much more useful if we could also copy (rather than just move) elements. That would avoid the duplication of HTML in the example above.

Bert Bos has proposed an alternative syntax, which allows both moving and copying elements into running heads. In the example below, h2 elements appear in their normal place in the document, but are also copied into running heads.

Why is float:bottom used with the footnote area? Floating footnotes to the footnote area, and then floating the footnote area itself, seems overly complex, given that implementations don’t allow the footnote area to float anywhere else. Note that some implementations do allow the footnote area to be absolutely positioned. ↵